Process for the production of precipitated calcium carbonates and product produced thereby

ABSTRACT

A process for producing precipitated calcium carbonate, including the steps of providing calcium hydroxyde, and carbonating the calcium hydroxyde with carbon dioxyde gas and communicating for a time sufficient to produce a calcium carbonate having at least about a 90 weight percent conversion to calcium carbonate and having a solids concentration of at least about 90 weight percent. Also, a process for producing precipitated calcium carbonate, including the steps of providing calcium hydroxide, carbonating the calcium hydroxide with carbon dioxide gas for a time sufficient to at least partially convert the calcium hydroxide to calcium carbonate, comminuting the at least partially converted calcium hydroxide, and sequentially repeating steps of carbonating and comminuting for a time sufficient to substantially convert the calcium hydroxide to calcium carbonate having at least about a 90 weight percent conversion to calcium carbonate and having a solids concentration of at least about 90 weight percent.

FIELD OF THE INVENTION

The present invention relates generally to a process for producing aprecipitated calcium carbonate (PCC) and a product produced using theprocess. More specifically, the process of the present inventionproduces high-solids precipitated calcium carbonates that can bemanufactured starting with lime or calcium hydroxide and addingsufficient water to produce a precipitated calcium carbonate (PCC)product containing a maximum of about 10 weight percent water without afiltering or drying step.

The calcium carbonate particles produced according to the method of thepresent invention are particularly useful as fillers for paper aspigments for coated paper, as pigments for paints, as impact modifiersin polymers and may find specific application in the food, nutrition,cosmetic, and pharmaceutical industries.

BACKGROUND OF THE INVENTION

Precipitated calcium carbonate (PCC) is manufactured via a series ofcontrolled chemical reactions. Precipitated calcium carbonate (PCC) iscommonly prepared by first slaking lime (CaO), commonly referred to asquicklime, by mixing with water to form an aqueous slurry of calciumhydroxide (“milk of lime”). This slurry is then reacted with carbondioxide gas to precipitate calcium carbonate. The aragonite form ofprecipitated calcium carbonate (PCC) has an orthorhombohedral shape thatcrystallizes as long, thin needles that when manufacturing PCC using thegas-slurry process described above, however, the result is a low-solidsslurry containing from about 10 weight percent to about 30 weightpercent PCC which must be dewatered by mechanical, thermal, and/or otherdrying means known in the art to produce a high solids PCC. Becauseprecipitated calcium carbonate (PCC) produced by the method described bythe present invention contains at least about 90 weight percent solids,this method requires smaller vessels and less energy than a process thatproduces 10-30 weight percent precipitated calcium carbonate PCC.

Thus, the manufacture of high-solids PCC requires significant capitaland energy, in addition to time, equipment, and labor costs, in order toincrease the solids concentration of the as-produced PCC from gas-slurryprocesses.

RELATED ART

International patent application WO 00/34182 teaches methods of treatinglime, particularly carbide lime, containing insoluble impurities usingan aqueous solution of a polyhydroxy compound to extract calcium ionsfrom the lime to achieve a higher solubility of lime in solution thanusing water alone. After removing insoluble impurities, a purifiedsolution of calcium ions remains that may be used for the production ofcalcium containing products.

U.S. Pat. No. 3,150,926 teaches a process for producing calciumcarbonate by carbonating a mechanically fluidized bed of lime, either inits oxide or hydrated form, to which an excess of water has been added.The excess water is necessary to maintain the temperature sufficientlylow to prevent overheating and consequent agglomeration or fusing duringthe exothermic hydration and carbonation stages. During hydration thetemperature is preferably maintained in the range of 125 Fahrenheit to220 Fahrenheit. When lime, such as that prepared by calcining limestone,is used as the starting material, various crushing, pulverizing andscreening steps are performed prior to hydration.

European Patent No. 0912238 teaches a process for producing inorganicand organic powders by precipitation from a liquid reaction mixture. Theprocess includes passing along a tubular reactor a segmented reactionflow comprised of discrete volumes of a reaction mixture separated bydiscrete volumes of a separating fluid which is substantially immisciblewith the reaction mixture. The process is particularly useful in thepreparation of oxalates, sulfides, and mixed sulfides. Additionalpossibilities include the synthesis of oxides, mixed oxides, carbonates,mixed carbonates, hydroxides, and hydroxycarbonates by precipitation orco-precipitation in aqueous or alcoholic media in the presence of ureawhich is heated to generate a precipitating anion.

SUMMARY OF THE INVENTION

A process is provided for producing precipitated calcium carbonate,including the steps of providing calcium hydroxide, and carbonating thecalcium hydroxide with carbon dioxide gas and comminuting, which is amilling action that exposes the unreacted calcium hydroxide allowing itto contact and react with the carbon dioxide gas stream to produce acalcium carbonate having at least from about 90 weight percentconversion of the calcium hydroxide feed to calcium carbonate and havinga solids concentration of at least about 90 weight percent.

Also provided is a process for producing precipitated calcium carbonate,including the steps of providing calcium hydroxide, reacting the calciumhydroxide with carbon dioxide gas to produce a calcium hydroxide/calciumcarbonate mixture, comminuting this mixture, and sequentially repeatingthe steps of reacting the calcium hydroxide with carbon dioxide gas andcomminuting the calcium hydroxide/calcium carbonate mixture until thismixture is substantially converted to at least about 90 weight percentcalcium carbonate having a solids concentration of at least about 90weight percent.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for the preparation of at leastabout 90 weight percent precipitated calcium carbonate (PCC). Typicalmoisture ranges of the as-produced materials generated by this processare, preferably, from about 3 weight percent to about 10 weight percentusing no filtering or drying steps. The method requires reacting calciumhydroxide that is either purchased or produced from lime with carbondioxide gas to directly synthesize precipitated calcium carbonate (PCC)of at least about 90 weight percent solids without filtering or drying.More specifically, according to the process of the present invention ahydrated lime feed is reacted with a carbon dioxide gas stream and,either simultaneously or alternately, comminuted to produce at leastabout 90 weight percent precipitated calcium carbonate (PCC). Hydratedlime is calcium hydroxide made by reacting lime with water. Hydratedlime can be produced from commercially available limes or purchased as araw material. The process of the present invention produces ahigh-solids PCC having a solids concentration of at least about 90weight percent based on the total weight of the product.

SLAKNG

Slaking, as defined here, means to react lime with water to producecalcium hydroxide and/or adjusting calcium hydroxide to a maximum ofabout 10 weight percent moisture. Preferably, the hydrated lime solidsconcentration is above about 90 weight percent and, most preferably,between about 90 weight percent to about 92 weight percent. Thishydrated lime feed solids concentration produces slaking temperatures ofup to 600 degrees Fahrenheit (315 degrees Celsius), while also providingenough water for carbonation to ultimately provide, with comminution, aproduct of at least about 90 weight percent precipitated calciumcarbonate (PCC).

The slaking is most preferably conducted to produce about 92 weightpercent slake solids at a temperature of up to about 600 degreesFahrenheit (315 degrees Celsius). Slaking is continued until conversionto a hydrated lime having a high-solids content is substantiallycomplete, preferably, being terminated when at least 90 percentconversion to calcium hydroxide having a solids content of from about 90weight percent to about 97 weight percent, and most preferably, about 92weight percent solids. For example, when mixing from about 50 pounds ofwater with about 75 pounds of lime, slaking is usually accomplished in aperiod of about 30 minutes to about 60 minutes to produce about 95-100pounds hydrated lime having from about 90 to about 97 weight percentsolids.

CARBONATION AND COMMINUTION

The calcium hydroxide contained in the hydrated lime is then subjectedto carbonation by reacting it with carbon dioxide gas to produceprecipitated calcium carbonate. Unlike conventional slurry processes forthe production of PCC, the carbonation step according to the presentinvention does not require any cooling of the carbon dioxide gas. Thenature of the carbon dioxide gas for the carbonation is not particularlycritical, the standard mixtures of carbon dioxide in either nitrogen orair as found in wet-scrubbed gases being satisfactory although purecarbon dioxide gas or liquid carbon dioxide can be used. The carbonationof the hydrated lime is continued until the conversion to calciumcarbonate is at least about 90 weight percent, i.e., from calciumhydroxide to precipitated calcium carbonate (PCC). Preferably, water isadded during carbonation to maintain the solids content of the mixtureat approximately from about 90 to about 97 weight percent. Mostpreferably, water is added to maintain from about 90 weight percent toabout 92 weight percent calcium hydroxide/calcium carbonate mixturesolids concentration during carbonation.

According to the present invention, comminution occurring eithersimultaneously or alternately with the carbonation step is performed inorder to expose the unreacted calcium hydroxide to achieve a high-degreeof conversion to calcium carbonate during the carbonation reaction. Asused herein, the term “comminution” means any process that shatters,cracks, fractures, breaks, or otherwise exposes the calcium hydroxidecomponent of the material being processed and includes, but is notlimited to, any milling, grinding, or pulverizing step for accomplishingthe same.

When comminution is to be performed alternately with carbonation, thecarbonation reaction may be performed using a pressurized vessel such asfor example, a pipe pressurized to 40 pounds per square inch with carbondioxide gas, to achieve, typically, greater than a 90 percent conversionof calcium hydroxide to calcium carbonate. The resultant calciumcarbonate/hydrated lime mixture is then sequentially milled by removingit from the pressurized pipe and, in the case of small batches, by handmilling the mixture using a mortar and pestle, coffee grinder, or othersuch like device. The mixture is again charged to the pressurized pipe,subjected to carbonation, removed, and milled with these steps beingsequentially repeated until conversion to calcium carbonate is at leastabout 90 weight percent. This method of repeatedly carbonating andcomminuting in sequential steps demonstrated a conversion to calciumcarbonate of at least about 90 weight percent.

Comminution may also be performed simultaneously with carbonation.Comminuting apparatus useful in performing this simultaneous reactioncan be a tumbler or ball mill that incorporates comminuting media ofvarious diameters and weights for milling/agitation during the reaction.An alternative apparatus useful in performing a continuous batchreaction is a mixer that has been retrofitted with a carbon dioxide gassupply and uses both comminuting media and, preferably, alsoincorporates a rotating scraping blade to prevent caking of the materialagainst the mixer wall during processing, thereby providing morethorough comminution. Regardless of the comminution apparatus utilized,the requisite degree of comminution is that needed to repeatedly exposethe interior of the unreacted hydrated lime feed to permit itsconversion to calcium carbonate during carbonation. By varying the medialoads, operating times, carbon dioxide gas concentrations, carbondioxide gas rates, or any combination of these factors, the degree ofcomminution can be adjusted to achieve exposure and conversion of theunreacted hydrated lime feed. The PCC conversion achieved using thissimultaneous carbonation and comminution accomplishes virtually completeconversion to a high-solids precipitated calcium carbonate product asdescribed in greater detail below.

The as-produced precipitated calcium carbonate may be utilized as suchfor fillers, dry coating applications, and plastics-productionadditives. The as-produced precipitated calcium carbonate havinghigh-solids may also be packaged and delivered to end-users for use infilling and coating applications. Alternatively, further finishing stepsmay be performed on the as-produced precipitated calcium carbonate toremove remaining inerts, such as, for example, magnesium andsilica-containing compounds, in order to produce high purity PCC'suseful in the production of paints, plastics, and healthcare products.

SPECIFIC EXAMPLES AND TABLES

The following non-limiting examples are provided to more specificallyteach and set forth particular embodiments of the present invention asenvisioned here. They are for illustrative purposes only, however, andare not to be construed as limiting the invention. It is recognized thatminor changes and alterations, such as for example a high-intensitymedia mill in conjunction with a concentrated carbon dioxide gas source,can be made to the process parameters and components that are notspecifically contemplated herein. However, to the extent any suchchanges or alterations do not materially change or affect the process orthe final product, it is to be understood that such changes also fallwithin the spirit and scope of the invention as defined by the claimsthat follow.

STARTING FEED MATERIALS AND SLAKING PROCESS

Starting feed materials used to produce precipitated calcium carbonatesaccording to the present invention were both hydrated lime havingcalcium hydroxide as the major component produced by slakingcommercially available lime sources (Hydrates 1 and 2) and hydratedlimes that are commercially available (Hydrates 3 and 4), the chemicalcompositions for which are listed below in Table 1. TABLE 1 STARTINGFEED MATERIALS Hydrate from Hydrate from Mississippi MississippiBeachville MR200 Rotary Mississippi Vertical Commercial CommercialCommercial Codex Commercial Lime Lime Hydrate Hydrate (Hydrate 1)(Hydrate 2) (Hydrate 3) (Hydrate 4) XRF DATA - X-Ray FluorescenceAnalysis CaO (weight percent) 73.46 73.93 74.65 74.81 LOI (weightpercent) 24.96 24.79 24.27 24.25 SiO₂ (weight percent) 0.94 0.42 0.540.53 MgO (weight percent) 0.62 0.78 0.41 0.37 TOTAL 99.98 99.92 99.8799.96 TGA/DTA DATA - Thermal Analysis Ca(OH)₂ (weight percent) 92.0 94.094.0 94.5 CaCO₃ (weight percent) 4.5 2.5 2.5 1.5 Free H₂O (weightpercent) 0.6 0.5 0.6 0.7 Misc. Losses (weight 0.4 0.4 0.2 0.3 percent)Mg/Si/Al/Fe (weight 1.71 1.37 1.12 1.05 percent) TOTAL 99.21 98.77 98.4298.05 Possible CaO(weight 1.32 1.39 2.11 2.46 percent) TRACE ELEMENTS (χ20 ppm) Al 320 258 389 329 Fe 341 432 445 258 K 29 104 49 22 Mg 23703470 1730 1520 Mn — 63 — — Na 123 118 124 121 P 48 — 51 39 Si 1700 10101970 2350 Sr 166 154 168 177 Ti — — — 22 V — — 24 — Zn — 22 — —

The Mississippi lime and hydrate materials referred to above areavailable from the Mississippi Lime Company, Ste. Genevieve, Mo., andthe Beachville lime materials referred to above are available fromCarmeuse Group North America, Beachville Plant, Ingersoll, Ontario,Canada. The hydrates identified above in Table 1 as Hydrates 1 and 2,respectively, were produced by slaking Mississippi and Beachvillecommercial limes. The Mississippi and Beachville limes having chemicalcompositions set forth in Table 1 above were slaked with water in aweight ratio of approximately 0.7 pounds of water per pound of lime.More specifically, to produce Hydrate 1, 75 pounds of lime was slaked byadding 53.1 pounds of water at 68 degrees Fahrenheit (20 degreesCelsius) over a period of 32 minutes to permit the exothermic slakingreaction to reach a reaction temperature up to about 600 degreesFahrenheit (316 degrees Celsius). This elevated temperature wasmaintained below where calcium hydroxide decomposition occurs,approximately at about 600 degrees Fahrenheit (316 degrees Celsius), andthen lowered to below 200 degrees Fahrenheit (93 degrees Celsius) whencomplete. The temperature profile of the slaking process is set forthbelow in Table 2: TABLE 2 SLAKING TEMPERATURE PROFILE Slaker TemperatureMinutes into Hydration Degrees Fahrenheit Degrees Celsius 0  68° F. (20° C.) 3 212° F. (100° C.) 16 476° F. (247° C.) 32 122° F.  (50° C.)

This slaking process described above was repeated using Beachvillecommercial lime starting feed to produce Hydrate 2 having the propertiesset forth in Table 1 above.

EXAMPLE SET 1 ALTERNATE CARBONATION AND COMMINUTION

In order to investigate the effect of high-pressure carbon dioxide gasduring these experiments, calcium hydroxide was charged to a sealed pipeand the pipe was raised to 40 pounds per square inch usinghigh-pressured carbon dioxide gas. The pipe's contents were allowed toreact for about five (5) minutes, the vessel was then depressurized andthe material from the pipe was analyzed.

Results showed that approximately about 10 weight percent conversion toprecipitated calcium carbonate (PCC) had occurred. This mixture wassubsequently hand-milled using a mortar pestle and/or a coffee grinderand recharged to the pipe where it was re-pressurized to 40 pounds persquare inch with carbon dioxide gas and allowed to react for anadditional five (5) minutes. The mixture was dumped from the pipe andre-analyzed and it was observed that approximately about an additional10 weight percent conversion to precipitated calcium carbonate (PCC) hadoccurred. This finding indicated that comminution was a critical elementfor sustaining conversion of the calcium hydroxide/PCC mixture. Table 3shows various examples where the mixture conversion to at least greaterthan about 96.6 weight percent precipitated calcium carbonate (PCC)occurred using this method. TABLE 3 PRESSURIZED PIPE CONFIGURATIONSAMPLE NO. 1 2 3 Hydrate Feed Hydrate 1 Hydrate 1 Hydrate 1 PercentSolids 91.9 percent 92.2 percent 92.4 percent Charge Weight 50 grams 150grams 100 grams Run Conditions Concentration 100 percent at 100 percentat 100 percent at Carbon Dioxide Gas 40 pounds per 40 pounds per 40pounds per (volume percent) square inch square inch square inch Numberof Runs × 10 × 5 minutes 17 × 5 minutes 16 × 5 minutes Carbonation Time(minute)/Run Comminution Mortar and Mortar and Coffee Apparatus PestlePestle Grinder Run Results Conversion (percent) 98.5 percent 99.0percent 96.6 percent Solids (percent) 94.5 percent 93.0 percent 97.6percent

The data in Table 3 demonstrate the high conversion rates to calciumcarbonate achieved when using comminution sequentially with carbonationon a hydrated lime feed having high-solids, i.e., above 90 weightpercent and most preferably between about 90 weight percent to 92 weightpercent solids. More specifically, Sample No. 1 produced using ahydrated lime feed of 91.9 percent solids achieved a 98.5 percentconversion to calcium carbonate. Sample No. 2 produced using a hydratedlime feed of 92.2 percent solids achieved a 99.0 percent conversion tocalcium carbonate. Sample No. 3 produced using a hydrated lime feed of92.4 percent solids achieved a 96.6 percent conversion to calciumcarbonate having 97.6 weight percent solids. Table 3 shows conversion ofcalcium hydroxide to precipitated calcium carbonate (PCC) of at least96.6 weight percent can be achieved by comminuting calcium hydroxidefeeds with carbonation including high-solids feeds.

EXAMPLE SET 2 SIMULTANEOUS CARBONATION AND COMMINUTION

A tumbler was manufactured using a twelve-inch long, twelve-inchdiameter pipe fitted with bolted flange-rings and endplates ofapproximately fourteen inches in diameter. Four equally spaced internalone quarter (¼) inch wide baffles were longitudinally mounted within thepipe. The endplates were provided with an inlet for a carbon dioxide gasfeed supply and an outlet for vapor removal. The tumbler was chargedwith 300 grams of hydrated lime produced from Mississippi commerciallime (i.e., Hydrate 1 described above) and various diameters and loadsof comminuting media except for a comparative example (Sample No. 4)with which no media was used.

The tumbler was placed on a laboratory twin-roll horizontal roller thatrotated the tumbler at 25 revolutions per minute, which for this tumblerapparatus, approached the critical speed beyond which the comminutingmedia would not tumble but stick to the tumbler wall. During rotation,the tumbler was supplied with 3.46 cubic feet per minute at 14.5 volumepercent carbon dioxide gas to perform the carbonation reactionsimultaneously with the comminution to achieve conversion of the calciumhydroxide to PCC, the process parameters and properties for which areset forth in Table 4 below.

To evaluate the use of commercially available hydrated limes,Comparative Sample No. 12 and Sample No. 13 also were prepared in thetumbler, described above, using Vertical/Codex calcium hydrate (i.e.,Hydrate 4, described above) having a chemical composition as set forthin Table 1 above, as the hydrated lime feed with the process parametersset forth below in Table 4. TABLE 4 DATA FOR COMMINUTION USING TUMBLERSample No. Comp Comp 4 5 6 7 8 9 10 11 12 13 Hydrate Feed Hydrate 1Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1Hydrate 4 Hydrate 4 Percent Solids 94.8 94.8 94.8 94.7 91.7 91.7 91.791.7 99.4 93.4 Charge Weight 300 300 300 300 300 300 300 300 300 300grams grams grams grams grams grams grams grams grams grams Media Charge¾ Weight — 7.58 7.58 7.58 7.58 — — 7.58 7.58 7.58 Inch (pounds) Volume(ml) — 1800 1800 1800 1800 — — 1800 1800 1800 ½ Weight — 6.61 11.61 6.616.61 6.61 6.61 6.61 6.61 6.61 Inch (pounds) Volume (ml) — 1540 2690 15401540 1540 1540 1540 1540 1540 ¼ Weight — — — — — 2.20 2.20 2.20 2.202.20 Inch (pounds) Volume (ml) Run Conditions Tumbler Speed 25 25 35.325 25 25 8 25 25 25 (rev. per minute) Gas 100 100 100 100 100 14.5 14.514.5 14.5 14.5 Concentration (volume percent) Gas Rate 0.5 0.5 0.5 0.50.5 3.46 3.46 3.46 3.46 3.46 Cubic feet per min. Run Time (Min.) 15 1515 30 30 30 30 30 30 30 Run Results Conversion 45.7 97.3 95.0 95.2 97.094.1 91.4 96.9 14.4 85.1 (percent) Weight (grams) 341 375 406 397 339310.0 386.3 381.9 207 410 Solids (percent) 92.6 89.5 89.2 90.8 90.5 91.291.0 90.9 97.7 93.7

The data in Table 4 demonstrate the high conversion rates to calciumcarbonate achieved when using comminution simultaneously withcarbonation on a hydrated lime feed having at least about 91 weightpercent solids. More specifically, Sample Nos. 5-8 produced using carbondioxide with a hydrate feed having from about 94.7 percent to about 94.8percent solids achieved conversions of from about 95.0 percent to about97.3 percent calcium carbonate with solids contents of about 90 weightpercent. This is in comparison with a 45.7 percent conversion of

Comparative Sample 4 achieved using no comminuting media with a hydratefeed having 94.8 weight percent solids.

Sample Nos. 9-11 run using various media sizes and loads at 8revolutions per minute and 25 revolutions per minute with a carbondioxide gas provided at 3.46 cubic feet per minute at a concentration of14.5 volume percent further illustrate the beneficial effect ofincreasing both the amount of media charge and tumbler speed to producePCC having higher conversions. More specifically, when beginning with ahydrate charge of 91.7 weight percent solids, a tumbler speed of 8revolutions per minute gave a lower conversion to PCC of about 91.4percent (Sample 10) when compared to an increased conversion of about94.1 percent (Sample 9) obtained using a tumbler speed of 25 revolutionsper minute with the same amount of media charge. Upon further increasingthe media charge with a tumbler speed of 25 revolutions per minute,about 96.9 percent conversion (Sample 11) was obtained. All threeSamples 9-11 produced PCC solids of about 91 percent.

The data also show that commercially available hydrated limes may alsobe used as the feed for the process of the present invention when thehydrated lime is provided with water in amounts preferably approachingfrom about 8 weight percent to about 10 weight percent. Specifically,Comparative Sample No. 12 produced using a hydrated lime feed havingabout 99.4 weight percent solids yielding about a 14.4 weight percentconversion while Sample 13 having lower hydrated lime solids of about93.4 weight percent exhibited a conversion to about 85.1 percent.

EXAMPLE SET 3-SIMULTANEOUS CARBONATION AND COMMINUTION

A commercial-grade mortar mixer manufactured by Stow Co. Binghamton,N.Y., was retrofitted with a carbon dioxide gas supply line and watersupply piping with water spray nozzles located inside the mortar mixervessel for providing a water spray for temperature maintenance and dustcontrol during carbonation. Hydrated lime feed and media were charged tothe mixer and the unit agitated and gassed to complete the reaction. Tohelp prevent caking of the mixture against the inside wall of the mortarmixer, rubber-tipped or stainless steel stationary wipers were alsoprovided inside the mortar mixer to continuously scrape the wall duringrotation of the mortar mixer vessel, as well as lift the media togenerate the comminution required during the reaction step.

More specifically, the mortar mixer was charged with 16 pounds and 24pounds of hydrated lime and 15-pounds, 25-pounds, and 50-pound loads of¼-inch diameter comminuting media. Water was provided at rates to removethe heat generated by carbonation while maintaining the reaction atoptimal solids concentration. A comparative example (Comparative SampleNo. 14) was also run using 32 pounds of hydrated lime feed with nocomminuting media. Carbon dioxide gas of concentrations from about 17.0percent to about 17.8 percent and a flow rate of from about 14.2 cubicfeet per minute were supplied at room temperature to the mixer forvarying times of from about 75 minutes to about 120 minutes to producehigh conversions of the slake to PCC of up to about 97.6 percent.

Samples 14-17 and 20-21 were produced using hydrated lime produced fromMississippi commercial lime (i.e., Hydrate 1, described above) whileSample 18 was produced using hydrated lime produced from Beachvillecommercial lime (i.e., Hydrate 2, described above) and Sample 19 wasproduced using Mississippi commercial hydrated lime (i.e., Hydrate 3,described above), the process parameters and properties for which areset forth in Table 5 below. Additionally, Samples 14-17 and 20 wereproduced using rubber-tipped wipers while Samples 18, 19, and 21 wereproduced using stainless steel wipers. TABLE 5 DATA FOR COMMINUTIONUSING MORTAR MIXER Sample No. Comp 14 15 16 17 18 19 20 21 Hydrate FeedHydrate 1 Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1 Hydrate 1Hydrate 1 Percent Solids 93.3 92.8 83.9 87.0 90.4 91.1 86.4 89.0 ChargeWeight 32 16 16 16 24 24 24 24 (pounds) Media Charge ¼ Weight — 15 15 1525 25 50 50 Inch (pounds) Volume (ml) — 4086 4086 4086 6810 6810 1362013620 Run Conditions Gas Concentration 17.8 17.5 17.0 17.7 17.2 17.217.2 17.2 (Volume percent) Gas Rate (cubic 14.2 14.2 14.2 14.2 14.2 14.214.2 14.2 feet per minute) Run Time (minute) 105 120 120 120 86 75 90 90Run Results Conversion 88.1 92.2 92.7 96.6 97.0 96.3 97.6 96.4 (percent)Solids (percent) 95.2 95.4 97.3 90.0 93.6 95.6 92.0 94.2

The data in Table 5 demonstrate the high conversion rates toprecipitated calcium carbonate as high as about 98 percent achieved whenusing comminution simultaneously with carbonation on a hydrated limefeed having a range of solids, including hydrated lime feeds havinghigher solids contents i.e., above 90 weight percent. More specifically,Sample Nos. 15-21 produced using increasing media loads at a constantrotational speed with carbon dioxide gas provided at a constant rate(14.2 cubic feet per minute) and at a relatively constant concentration(17.0 volume percent to 17.7 volume percent) further illustrate thebeneficial effect of increasing both the amount of media charge toproduce PCC having higher conversions. More specifically, Samples 18-21having increased media loads of 25 pounds and 50 pounds exhibit higherconversions to PCC (96.3 percent to 97.6 percent) than ComparativeSample 14 which used no media and yielded the lowest conversion of about88 percent.

For media loads of 15 pounds of one quarter (¼) inch diameter mediacharges, Samples 15 and 17 show increasing conversion at lower finalsolids concentrations after run times of 120 minutes. For 25-pound mediacharges, Samples 18 and 19 show that maintaining lower solids increasesconversion. Moreover, Samples 20 and 21 which used a batch size of 1.5times that of Samples 15-17 with a media charge of more than three timeshigher (50 pounds., ¼″ diameter) show increasing conversion productivityat shorter gas times of 90 minutes verses 120 minutes. Moreover, Samples17-21 having more media and/or lower final product moisture all havehigher conversions than the no or low media cases of Samples 14-16.

Thus, the process according to the present invention produces ahigh-solids PCC having at least a 90 weight percent solids using areaction step that requires minimal dewatering or drying therebyeliminating the need for large filters and dryers. By performingsufficient comminution either during the addition of carbon dioixide gasto calcium hydroxide or alternately adding carbon dioxide gas to calciumhydroxide followed by comminution and recharging with gas and repeatingthis cycle demonstrates that conversions to calcium carbonate of about100 weight percent precipitated calcium carbonate (PCC) may be achieved.By eliminating the extensive drying steps typically performed forgas-slurry processes, the process of the present invention simplifiesthe process for producing PCC, thereby, providing low-cost PCC for usein fillers, coating-grade slurry applications, dry-coating applications,plastics-production additives, as well as for use in producing, withminimal additional finishing steps, PCC for paints, plastics, andhealthcare products.

Using materials produced by the process according to the presentinvention (i.e., rather than a conventional slurry) as the starting feedfor producing high solids PCC or further modified PCC products permitsthe use of small-scale equipment for post-processing while reducingstorage and transportation requirements.

Moreover, unlike conventional PCC production processes that utilizeslurries, the process provides significant additional operatingadvantages, among which are ability to use carbon dioxide gas supplywithout cooling, cutting water consumption and permitting the use of alow energy compressor to deliver the gas to this process. Additionally,the process according to the present invention reduces the wet-waste anddisposal costs associated therewith while eliminating a majority of thewater typically used for slaking.

While embodiments and application of this invention have been shown anddescribed, it will be appreciated by those skilled in the art thatmodifications and embodiments are possible without departing from theinventive concepts herein described. For example, although embodimentsare shown and described above with respect to specific gas-contactingand comminuting apparatus, it will be apparent to those skilled in theart that other similar devices may be employed to simultaneously oralternately expose the unreacted feed during carbonation to effect ahigh slake conversion according to the process of the present invention.Gas-contacting apparatus useful in this regard can include variouscommercially available mills that have been retrofitted with a gassupply to permit carbonation and comminution to be simultaneouslyperformed. Exemplary comminuting apparatus in this regard include airclassifying mills, hammer mills, jet-mills, pin mills, disc mills,colloid mills, agitated ball mills, sand mills or other mills known inthe art.

Other materials handling devices such as blenders, conveyors, dryers,and other vessels that have been retrofitted with a gas supply may alsobe used as gas contactors for practicing the process according to thepresent invention. Exemplary blenders in this regard includesingle-cone, double-cone, “V”-cone, and continuous blenders, and cementmixers. Exemplary conveyors in this regard include single-screw ormultiple screw conveyors that may also be provided in a helical-screw orfluted-shaft configuration. Exemplary dryers in this regard includespray, flash, rotary, tunnel, and tray dryers. Exemplary vesselconfigurations in this regard include those having a cylindrical,polygonal, oval, and spherical cross-sections. When used in combinationwith separate comminuting apparatus, these materials handling devicescan be used to perform alternate carbonation steps as described indetail above. By providing and using comminuting media in thesematerials handling devices, carbonation and comminution can be performedsimultaneously. Moreover, additional internal agitation (e.g., stirringblades, impellers, and internal baffles) may also be employed withblenders, dryers, and vessels where physically feasible to furtherenhance the carbonation reaction.

Moreover, although the examples shown and described above with respectto smaller batch processes using specific hydrated lime feeds, it willbe apparent to those skilled in the art that these processes may beprovided as full-scale batch or continuous reactions while using feedsfrom other lime or hydrated lime sources. Additional down-streamfinishing steps including drying, classifying, milling, surfacetreatment, may also be employed to achieve the desired final productcharacteristics.

Therefore, it is intended that the appended claims cover all suchmodifications and embodiments that fall within the true spirit and scopeof the present invention.

1. A process for producing precipitated calcium carbonate, comprisingthe steps of: (a) providing calcium hydroxide; and (b) carbonating thecalcium hydroxide with carbon dioxide gas and comminuting for a timesufficient to produce a calcium carbonate having at least about a 90weight percent conversion to calcium carbonate and having a solidsconcentration of at least about 90 weight percent.
 2. The processaccording to claim 1, wherein the calcium hydroxide provided is at leastabout 90 weight percent solids with water present in an amount of up toabout 10 weight percent.
 3. The process according to claim 2, whereinthe calcium hydroxide provided is about 92 weight percent solids.
 4. Theprocess for producing precipitated calcium carbonate according to claim1, wherein the calcium hydroxide provided in step (a) is produced by thesteps comprising: i) mixing calcium oxide and water in amountssufficient to react to form calcium hydroxide substantially free ofwater; and ii) maintaining the mixture at an elevated temperature for atime sufficient to hydrate the calcium oxide to form calcium hydroxidehaving at least about 90 weight percent solids and water present in anamount of up to about 10 weight percent.
 5. The process according toclaim 4, wherein the step of maintaining the mixture at an elevatedtemperature is performed at a temperature of up to about 600 degreesFahrenheit for a time sufficient to hydrate the calcium oxide to formcalcium hydroxide having at least about a 95 weight percent conversionto calcium hydroxide.
 6. The process according to claim 5, wherein thestep of maintaining the mixture at an elevated temperature is performedfor a time sufficient to hydrate the calcium oxide to form calciumhydroxide having at least about a 98 weight percent conversion tocalcium hydroxide.
 7. The process according to claim 1, wherein thesteps of carbonating and comminuting are performed until at least a 95weight percent conversion to calcium carbonate is achieved.
 8. Theprocess according to claim 1, wherein the steps of carbonating andcomminuting are performed until at least a 97 weight percent conversionto calcium carbonate is achieved.
 9. A process for producingprecipitated calcium carbonate, comprising the steps of: (a) providingcalcium hydroxide; (b) carbonating the calcium hydroxide with carbondioxide gas for a time sufficient to at least partially convert thecalcium hydroxide to calcium carbonate; (c) comminuting the at leastpartially converted calcium hydroxide; and (d) sequentially repeatingsteps of carbonating and comminuting for a time sufficient tosubstantially convert the calcium hydroxide to calcium carbonate havingat least about a 90 weight percent conversion to calcium carbonate andhaving a solids concentration of at least about 90 weight percent. 10.The process according to claim 9, wherein the calcium hydroxide providedis at least about 90 weight percent solids with water present in anamount of up to about 10 weight percent.
 11. The process according toclaim 10, wherein the calcium hydroxide provided is about 92 weightpercent solids.
 12. The process for producing precipitated calciumcarbonate according to claim 9, wherein the calcium hydroxide providedin step (a) is produced by the steps comprising: i) mixing calcium oxideand water in amounts sufficient to react to form calcium hydroxidesubstantially free of water; and ii) maintaining the mixture at anelevated temperature for a time sufficient to hydrate the calcium oxideto form calcium hydroxide having at least about 90 weight percent solidsand water present in an amount of up to about 10 weight percent.
 13. Theprocess according to claim 12, wherein the step of maintaining themixture at an elevated temperature is performed at a temperature of upto about 600 degrees Fahrenheit for a time sufficient to hydrate thecalcium oxide to form calcium hydroxide having at least about a 95weight percent conversion to calcium hydroxide.
 14. The processaccording to claim 13, wherein the step of maintaining the mixture at anelevated temperature is performed for a time sufficient to hydrate thecalcium oxide to form calcium hydroxide having at least about a 98weight percent conversion to calcium hydroxide.
 15. The processaccording to claim 9, wherein the steps of carbonating and comminutingare performed until at least a 95 weight percent conversion to calciumcarbonate is achieved.
 16. The process according to claim 15, whereinthe steps of carbonating and comminuting are performed until at least a97 weight percent conversion to calcium carbonate is achieved.
 17. Thecalcium carbonate product produced-by-the-process of claim
 1. 18. Thecalcium carbonate product produced-by-the-process of claim
 4. 19. Thecalcium carbonate product produced-by-the-process of claim
 9. 20. Thecalcium carbonate product produced-by-the-process of claim 12.